Many types of electrical devices, such as but not limited to batteries, capacitors, and hydrogen-storage devices, include electrodes, plates, or analogous structures made of a carbonaceous material in the form of graphite or carbon (e.g., “activated carbon”). In many instances the efficacy of these structures is a function of the surface area of the structure.
For example, the “anode” in a conventional lithium-ion (Li-ion) battery cell is typically made of graphite, while the “cathode” is typically made of a metal oxide. The materials of both electrodes are ones into which, and from which, lithium ions can migrate. The process of movement of lithium ions into an electrode material is called “insertion” or “intercalation.” The process of movement of lithium out of an electrode material is called “extraction” or “deintercalation.” Since each lithium ion takes up a finite amount of space, the rate and number of ions that can be intercalated in a given volume of electrode material is a function of the surface area, on a nanometer dimensional scale, of the electrode material. A similar principle is applicable to carbon electrodes, plates, or other structures used in other applications such as supercapacitors.
Since the advent of practical nano-technology, forming nano-structures on an active surface is oftentimes considered for increasing the surface area, especially on a nanometer scale, of the surface. Heretofore, carbon surfaces (including doped carbon surfaces) with nano-sized features have been difficult to fabricate because the conventional technique for forming the nano-sized features involves use of a sacrificial alumina template. See, e.g., Hulteen et al., NanoStructured Materials 9:133-136 (1997). The “template” used in the Hulteen method is not really a template at all because it does not provide deliberately formed nano-concavities. Rather, reliance is placed on the natural random porosity of alumina, wherein each unit of alumina has its own natural, unique, random labyrinth of pores, and there is no control from one unit of alumina to the next. In the method, the pores in the alumina are filled with a polymer. Then, the filled template is subjected to a graphitizing condition to convert the polymer to graphite. After carbonization the alumina template is etched away, leaving a carbon structure behind. Depending upon the sizes and shapes of the pores, some of the carbon structure can be considered “nano-structures.”
A key disadvantage of the Hulteen and similar techniques is that, since the alumina template is destroyed during use, a new template is required for every nano-structured unit of carbon that is prepared, which is usually a very small unit. Also, this conventional method is very time-consuming, resource-wasteful, expensive, unreliable, too variable, and not amenable to mass-production. Other disadvantages include: (1) The template-etching step is very aggressive and results in substantial chemical and physical damage to any carbon nano-structures that were formed by the alumina. For example, the carbon nano-structures are not ordered, but rather are poorly formed and poorly defined, and it is impossible to form an ordered array of similarly sized and shaped nano-structures. (2) The template-etching step is very difficult to control, leading to highly variable and often unpredictable results. (3) Bulk alumina has voids and thus behaves as a filter material. Filling concavities in an alumina template with a polymer or polymer-forming substance results in molecules of the polymer extending into the voids and migrating throughout the template, including to other surfaces thereof. These penetrated molecules of polymer, when subjected to carbonization conditions, form random nano- and micro-structures that have poor structural fidelity. (4) To etch silica within a reasonable time, the surfaces are exposed to sodium hydroxide, which is complicated to perform on delicate graphite structures and tends to degrade them. Also, even a “reasonable time” for etching is impracticably long, usually several hours or more. (5) Having to etch away the unit of alumina after each use requires a large amount of toxic chemicals and extra procedures, and hence entails high cost. (6) The randomness of the outcome, poor controllability, and poor predictability of the method makes it extremely difficult or impossible to “tune” the process of making nano-architectured graphite, e.g., tuning by customizing process ingredients (including dopant(s)), temperatures, pressures, and other parameters to achieve a particular result (e.g., a particular array of particularly sized and shaped nano-features for producing a desired activity) on a consistent basis. (7) Including additives such as dopant(s) in the polymer is practically impossible because most additives are destroyed in the alumina-etch process. (8) Whatever survives the template-etching process must be cleaned, which is also an aggressive process that degrades the product while adding extra process time and cost. (9) Heating a polymer-filled alumina template to a practical graphitization temperature (above 2000° C.; above 2500° C. for high-quality graphite) results in melting of the alumina (melting point at atmospheric pressure is 2070° C.), which destroys the alumina and everything in it and on it. (10) Use of an alumina template inherently cannot form regular arrays of particularly shaped nano-features. Hence, the product is not suitable for any application in which a regular array(s) of nano-features is desirable or necessary.
Certain applications of carbon electrodes would benefit greatly if it were possible to produce electrodes exhibiting particular activities (e.g., specific capacities) that are enhanced more than currently achievable. For example, carbon electrodes with increased specific capacity would be particularly useful in devices requiring fast charging/discharging rates, such as lithium batteries and battery-capacitor hybrid devices. Certain additives (e.g., silicon) are known to have specific capacities greater than carbon or graphite alone, but heretofore it has not been practical to incorporate additives (as “dopants”) in carbon electrodes in a way that achieves a reliable and significant increase in the specific capacity of the electrode.
In view of the various deficiencies of conventional methods as summarized above, there are no known reports of making, for example, carbon battery electrodes or capacitor electrodes by such methods. There are also no known reports of such electrodes in which the carbon includes a dopant(s) for achieving enhanced performance or other desired activity level.
In this era of increased demand for more miniaturized electrical devices, the advantages of making smaller power sources are readily apparent. For example, increasing the number of ion-intercalations per unit volume of an electrode material allows correspondingly smaller electrodes, plates, or other structures to be made that exhibit the same or greater performance than their conventional counterparts. Reducing the size of electrodes, plates, and the like also allows correspondingly smaller devices (e.g., batteries, supercapacitors) to be made.
Hence, there is a need for improved methods for making carbon structures, whether doped or non-doped, suitable for use in electrical devices (e.g., batteries, supercapacitors) of progressively smaller size, without sacrificing performance. There is also a need for improved methods for fabricating nano-architectured carbon structures efficiently, consistently, and at high throughput for cost-effective manufacture of high-performance batteries, supercapacitors, and other electric-power devices.